C9 - Energy, Power and Resistance Flashcards
What is potential difference?
Also known as voltage, it’s the measure of the transfer of energy by charge carriers (electrons).
What equation links voltage, work done and charge?
V = W/C
Therefore one volt is equal to 1 J/C
What is e.m.f?
Electromotive force is the energy transferred from chemical to electrical energy per unit charge.
It’s used to describe when work is done on electrons.
How is electromotive force calculated?
e = W / Q
E.m.f. = work done / charge
What is an electron gun?
An electrical device used to produce a narrow beam of electrons.
They can be used for ionisation and have precise kinetic energies.
How does an electron gun work?
A small metal filament is heated by an electric current as a source of electrons.
The electrons in this piece of wire again kinetic energy.
Some of them gain enough energy to escape from the surface of the metal.
This process is called thermionic emission.
If the heated filament is placed in a vacuum and a high voltage is applied between the filament and an anode, the filament acts as a cathode and the freed electrons accelerate towards the anode daily kinetic energy.
If the anode has a small hole in it then the electrons in line with this hole can pass through it creating a beam of electrons with a specific kinetic energy.
What is work done on an electron equal to?
Work done on electron = gain in kinetic energy
eV = 1/2mv2
The greater the voltage, the more energy is transferred to the electrons and so the faster they move.
What is resistance?
A property of the components calculated by dividing the voltage across it by the current, measured in ohms.
What is voltage?
Potential difference is energy transferred from electrical energy into other forms per unit of charge
What is ohms law?
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the voltage across its ends.
If temperature increases, resistance increases and the cations of the metal are moving more.
How is the IV graph for a fixed resistor plotted?
It is a straight line through the origin where current and voltage are directly proportional.
They ohmic conductors.
How is the IV graph for a filament lamp plotted?
It is a positive and negative curve through the origin.
How is the IV graph for a diode plotted?
It is a curve in one direction (and zero in the other).
What is resistivity?
The property of the material (measured in ohmic metres) defined as the products of the resistance of a component made of the material and it’s cross-sectional area divided by its length.
What affects resistance?
The material of the wire
The length of the wire
The cross sectional area of the wire
